Coiled Tubing (CT) is frequently used in the completion of long, horizontal oil and gas wells. These wells have horizontal sections ranging from about 2,000 to about 10,000 feet and they are increasing rapidly in length over time. The operating conditions require CT with an outer diameter (OD) of at least about 2.000 inches, wall thickness (WT) of at least about 0.156 inch, and steel tubing with a specified minimum yield strength (SMYS) of at least about 90,000 psi.
As those of skill in the relevant arts will readily appreciate, CT is a consumable that is sold in discrete lengths known as “strings,” and each CT string has a finite lifespan. The life of the CT string in long horizontal wells is primarily governed by the durability of the helical strip joining weld known as the “bias weld.”
A typical CT string used in such wells will have between about 10 and about 20 bias welds within it. Bias welds are the weakest point of the CT string and are modeled to have a life that is about 80% of the life of areas of the CT string without bias welds. The basis for modeling 80% life was determined in a 1995 joint industry study on plastic fatigue when CT strings were used only in vertical wells.
However, long, horizontal wells have operating conditions that create unique stresses on CT strings and bias welds in particular. These stresses, such as helical and sinusoidal buckling, are in addition to the typical plastic strain CT strings encounter in vertical wells. To improve the economics of CT usage in long, horizontal wells it is imperative that the life of bias welds is increased.
The life of a bias weld is controlled by many variables, but WT has the largest impact. For a typical string used in long, horizontal wells, a 17% increase in WT leads to a 78% increase in bias weld life. As the WT increases so does the overall weight of the CT string. The units that carry and deploy CT strings have structural and/or regulatory weight limits.
To stay within weight limits, CT service companies must limit either WT or CT string length. Decreasing the CT string length reduces the market potential and profit margin for that CT string. For example, in the United States, a 15,000 foot CT string may only be utilized a third as much as a 19,000 foot CT string. Therefore, most service companies sacrifice bias weld life for longer CT strings.
There is, therefore, a long-standing yet unmet need for improved methods and means of increasing the life of bias welds in coiled tubing used in long, horizontal wells, which are still economical on a commercial scale.